Process for rehabilitating internally reinforced concrete by removal of chlorides

ABSTRACT

A process for rehabilitation of internally reinforced concrete which comprises the temporary application of an adherent coating of an electrolytic material, formed of cellulosic pulp to surface areas of the concrete. Distributed electrode means, preferably a ferrous metal wire grid, is embedded in the adherent coating. Voltage sufficient to develop a current of about 1-5 amps/m 2  is applied between the reinforcement and the distributed electrode, to cause migration of chloride ions from the concrete into the electrolytic coating. When the chloride content of the concrete has been reduced to a desired level, the voltage is discontinued and the electrolytic coating and distributed electrode are removed. Particularly for tensioned reinforcement, the process monitors polarization of the tensioned steel to prevent hydrogen embrittlement.

The present application is a division of earlier application Ser. No.366,204, filed Jun. 9, 1989. Application Ser. No. 366,204 was acontinuation-in-part of prior application Ser. No. 352,266, filed May16, 1989. The last mentioned application was a continuation-in-part ofprior application Ser. No. 100,834, filed Sep. 25, 1987, now U.S. Pat.No. 4,832,803, granted May 23, 1989.

BACKGROUND AND SUMMARY OF INVENTION

The present invention is directed to a method for rehabilitatinginternally reinforced concrete by removal of chlorides.

One of the serious problems in connection with the maintenance ofreinforced concrete is the matter of corrosion of the internalreinforcement. In many cases corrosion of the reinforcement is caused bythe chloride contamination. This may result from the gradual absorptionof chlorides over a period of time or, in some cases, from theincorporation of chlorides into the original concrete to acceleratesetting. Conventional techniques for repairing chloride-contaminatedconcrete involve physical removal of the contaminated material andreplacement with fresh concrete. This is obviously a costly anddisruptive solution, at least with respect to vertical and overheadsurfaces.

It has been proposed heretofore to employ electrolytic techniques forremoving chlorides by ion migration. An article by J. E. Slater,Materials Performance, 1976, pp 21-26, describes such a method, whichinvolves applying an electric potential between internal reinforcementand a surface electrode submerged within a liquid electrolyte containedon the surface of the concrete With the surface electrode forming thepositive pole of the electric field, the negative chloride ions withinthe concrete are caused to migrate through the concrete and out into theelectrolyte where they are oxidized to chlorine gas on the positiveelectrode or react chemically with components in the electrolyte.

The experiments described in the Slater article were carried out onchloride contaminated bridge decks, where the reinforcement hadcorroded. The bridge deck area was divided into sections of about 3.5 m²that were individually treated. These sections were provided with sealeddams for containing a liquid electrolyte solution. The electrolyte usedwas a calcium hydroxide solution, with and without ion exchangers.Slater used voltages of between 100 and 120 V, and the current variedbetween 28 and 100 amps per section. Slater was able to remove up to 90%of the chlorides in the concrete within a 24 hour period. Where theelectrolyte was used without ion exchangers, chlorine gas developed onthe platinized titanium electrodes and was released as free chlorinegas.

The Slater technique has not been commercially successful, for severalreasons: For one, the safety considerations are substantial whenoperating at voltages as suggested by Slater. More importantly, theSlater procedure is useful only for removing chlorides from the uppersurface of a horizontal slab. However, the more conventional method ofremoving the concrete is relatively simple and inexpensive when dealingthe an upwardly facing horizontal slab. The Slater method may well bemore costly than conventional techniques.

The present invention provides an economical electrolytic method ofremoving chlorides from contaminated reinforced concrete that can becarried out safely and with reasonable energy requirements and which,importantly, can be utilized on vertical and downwardly facing surfaces.In accordance with one important aspect of the invention, the processutilizes an electrolytic material in the form of an adherent coatingthat can be applied to, and will adhere to, vertical surfaces or evendownwardly facing surfaces. A distributed electrode means is embeddedwithin the adherent coating and forms the positive terminal of theelectrical system. When the process has been completed, that is, whenthe level of chloride contamination has been reduced to a desired level,both the adherent coating and the electrode means are removed from theface of the concrete. In this respect the process of the inventiondiffers markedly from cathodic protection systems, for example, wheresystems are permanently installed for continuously maintaining anelectric potential between internal reinforcement and surface electrodemeans.

In our above-mentioned patent application, a material such as retardedgunite is disclosed as one of the materials suitable for use as anadherent electrolyte coating having adequate conductivity and beingremovable upon completion of the procedure. In an especiallyadvantageous procedure according to the invention, the adherentelectrolytic coating material is formed of a mixture of cellulosic pulpfiber and water or other solution, which is self-adherent to the surfaceof the concrete. The pulp fiber, which is advantageously derived fromre-cycled newsprint, is mixed with the liquid solution at the outlet ofa spray nozzle, and the fiber-liquid pre-mix is sprayed onto the surfaceof area to be treated. The surface of the concrete draws some of thesolution from the sprayed mixture and causes the mixture to adheretenaciously to the concrete surface.

Pursuant to the invention, the concrete area to be treated is tested bythe taking of cores and testing for chloride content. From these initialtests, the approximate time required to achieve a desired level ofchloride reduction can be estimated. The treatment can be continueduntil approximately the estimated time has passed, after which a furtherset of cores may be taken to establish the final processing conditions.

For a more complete understanding of the features and advantages of theinvention, reference should be made to the following description ofpreferred embodiments and to the accompanying drawing.

DESCRIPTION OF THE DRAWING

FIG. 1 is a fragmentary elevational view of a concrete wall beingprepared for treatment according to the invention, with parts brokenaway to show certain additional details.

FIG. 2 is an enlarged fragmentary cross sectional view as takengenerally on line 2--2 of FIG. 1.

FIG. 3 is a simplified representation of a typical voltage-versus-timecurve of a reference voltage monitored to determine the condition of thereinforcing steel of the concrete under treatment.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawing, the reference numeral 10 designates in ageneral way a body of a steel-reinforced concrete structure, which maybe in the form of a vertical wall or overhead structure. The concretewall typically includes a series of embedded steel reinforcing bars 11of conventional type.

Pursuant to the teachings of our parent application, a concretestructure 10, 11 that has become contaminated with chlorides, can berehabilitated by applying an adherent electrolytic coating 12 to onesurface of the structure, preferably that which has become mostcontaminated. A temporary distributed electrode 13, advantageously inthe form of a grid-like structure of conductive wire, is embedded withinthe electrolytic coating 12. A source 14 of direct current voltage isconnected between the internal reinforcing bars 11 and the temporaryelectrode structure 13. Over a period of time, an electrolytic migrationof chloride ions from the internal regions of the concrete wall,adjacent to the reinforcing bar 11 and into the electrolytic medium 12,is effected by the electric field. In accordance with broader aspects ofthe invention, after a predetermined period of treatment, wherein thechloride content of the concrete has been sufficiently reduced, thevoltage source 14 is disconnected, the electrode 13 and electrolyticmedium are removed, and, typically, the outer surface 15 is coated witha sealant layer (not shown) to inhibit future infusions withchloride-containing compositions.

Significantly, the external electrode means 13 is of a distributednature. Most advantageously, this can be realized by utilizing agrid-like structure consisting of a first set of wires 16 arranged atright angle to a second set of wires 17, with the wires being spotwelded or otherwise joined at the intersecting points. A particularlyadvantageous form of electrode structure 16 is comprised of a grid ofwire 16, 17, as illustrated, where the wires are approximately six mm indiameter and are constructed to form a grid with openings of about 10-15cm on a side. The specific form of electrode structure can be variedsignificantly, of course, as long as the electrode structure isdistributed relatively uniformly throughout the entire surface areabeing treated.

In many electrolytic systems for the treatment of concrete, such ascathodic protection systems, for example, or the system of the Slaterarticle heretofore mentioned, the external electrode system is formed ofa material such as platinized titanium, which is non-reactive to themigrating chloride ions. In appropriate circumstances, the distributedelectrode used in the process of the invention may be formed of similarmaterials. Such materials have a disadvantage, however, of releasingfree chlorine gas to the surrounding ambient, unless some kind of ionexchange material is utilized. As will be understood, the release ofchlorine gas can pose safety hazards, unless ventilation is adequate.The utilization of ion exchange materials, on the other hand, involvesadded expense.

In a preferred practice of the invention, the external electrode 13 isformed of steel. During operation of the process, the free chlorine ionsreact with the steel causing the steel to corrode. This substantiallyminimizes any release of free chlorine gas. Over a period of time, thecorrosion products will reduce the cross section of the individualwires, and it typically will become necessary to increase voltagesomewhat to maintain desired current flow densities. In certain cases,where the chloride contamination problem is particularly severe, theformation of corrosion products may actually cause discontinuities inthe distributed electrode structure, in which case the electrode 13should be replaced.

In a particularly advantageous practice of the invention, the wire gridelectrode 13 is supported spaced slightly away from the front face 15 ofthe concrete structure, by means of battens 18, typically in the form ofwooden strips about 2 cm on a side. As shown in FIG. 1, the battens 18may be attached to the face of the structure in the form of a large gridwhich may, for example, be about two meters on a side. The distributedelectrode structure 13 is then secured to the battens 18, by stapling orother suitable means, so that the external electrode is supported inappropriately spaced relation to the surface 15 of the concretestructure.

The electrolytic medium 12 may, in principle, be formed of anyself-adherent material capable of retaining moisture and, in suchcondition, having an adequate level of conductivity. As described in theparent application, this may include certain forms of grout, retarded soas to not set up during the course of the treatment. Moreadvantageously, however, the electrolytic medium comprises apapier-mache-like cellulosic material formed of a mixture of cellulosicpulp in water or other solution, such as calcium hydroxide solution. Theprocedure advantageously utilizes a conventional form of papermakingpulp, preferably but not necessarily formed of a recycled paper Thecellulosic pulp material is applied to the surface of the concretestructure by being mixed with the solution in a mixing nozzle 19 andbeing sprayed upon the surface of the concrete structure as the mixedmaterials emerge from the nozzle 19.

Desirably, the concrete structure is prepared for the application of thecellulosic medium by initially being tested for humidity level, andadjusted if necessary. This can be accomplished by forming a recess inthe concrete in a suitable location and then sealing off the outlet ofthe recess long enough for the relative humidity level of the air in therecess to stabilize. The relative humidity of this air is then measuredby conventional instrumentation. If the humidity level is 90% or less,it is desirable to spray the outer surface of the concrete structurewith water to the point of saturation. After a short period of surfacedrying, the mixture of cellulose pulp and water or other solution issprayed over the surface of the moist concrete. Because of the porouscharacter of the concrete, the cellulosic pulp-water or pulp-solutionmixture adheres strongly to the surface 15, because of the tendency forthe liquid of the pulp mixture to be drawn into the pores of theconcrete.

Desirably, the pulp-solution mixture is comprised of chopped pulp mixeduniformly with the solution in ratios of from about 2.7 - 1 to about4.0 - 1 of water or other solution per/kg of dry fiber. The twomaterials are joined at mixing nozzle 19, which is supplied with thepulp fiber entrained in air through an inlet tube 22, where it is joinedwith the liquid component, supplied through a tube 23. The pulp-solutionmixture quickly sets up in the form of a papier-mache-like material,which is self-adherent to the concrete and has a high degree ofcoherency with itself. Advantageously, a first layer of thepapier-mache- like material is sprayed onto the concrete surface to adepth approximately equal to the thickness of the wood battens 18 (e.g.,about 2 cm). After this first layer has been applied, the distributedelectrode grid 13 is attached to the exposed surfaces 24 of the battens,overlying the just-applied layer of the paper-mache. Thereafter,additional pulp-solution mixture is sprayed over the top of thedistributed grid structure, to provide a total layer of approximately4-5 cm in thickness.

The papier-mache-like material forming the electrolytic medium 12,because of its contained moisture content, has a sufficiently highdegree of conductivity to enable the process to be carried outadvantageously. Thus, the voltage source 14 may be connected to thesystem as soon as the pulp-liquid material has been applied in themanner described. Of course, it is necessary to maintain a certain levelof moisture in the papier-mache-like medium 12, and this is accomplishedby spraying the surface of the electrolytic medium 12 as often asnecessary. Usually, twice per day is adequate.

Pursuant to the invention, the voltage 14 is maintained until thechloride level within the concrete structure has been reduced to anacceptable level. Normally, core samples are taken at selected pointsbefore commencement of the procedure and, from the chloride content asmeasured from these initial core samples, the approximate requiredduration of the rehabilitation procedure may be estimated. As thatestimated period approaches, a further set of core samples may be taken,if desired, in order to establish with a higher degree of accuracy theremaining treatment required to bring the chloride level of concretedown to a predetermined, satisfactory level.

In a typical case, the voltage applied by the voltage source 14 isadjusted as necessary to maintain a current flow density between theinternal and external electrode systems of approximately one-five amp/m²of concrete surface. Usually, however, voltages are in all eventsmaintained at 40 volts or below for safety considerations.

Pursuant to one aspect of the invention, provision can be made formonitoring the condition of the reinforcing steel to avoid itspolarization over time. Especially where the reinforcing steel ismaintained under tension, for example, in certain types ofpost-tensioned or pre-tensioned concrete structures, it would notnormally be considered to utilize a procedure, such as describedhereinabove, for removal of chlorides, because of the danger of hydrogenembrittlement of the steel in tension. Thus, as the process continues,the reinforcing steel gradually becomes polarized. When the polarizationreaches a critical level, which may occur within a couple of weeks timein a typical process, evolution of hydrodgen is favored, and steel whichis in tension may become subject to hydrogen embrittlement. Suchconditions obviously would be highly detrimental to a tensionedstructure.

In a preferred practice of the invention, the condition of the internalreinforcing steel is monitored periodically. When the polarizationreaches a danger level, the procedure can be discontinued long enoughfor the polarization to dissipate or the current flow may be reversedfor a short period of time in order to dissipate the polarization at anaccelerated rate.

To advantage, monitoring of the polarization is accomplished by the useof a reference half cell 25, which is embedded in the concrete, closelyadjacent the reinforcing bar. When the voltage developed between thereinforcing bar and the reference half cell (hereinafter referred to asreference voltage) reaches a predetermined level, indicating a criticaldegree of polarization, the desired process modifications (e.g., voltageinterruption or voltage reversal) may be carried out. By way of example,where the reference half cell 25 is a copper-copper sulphate cell, avoltage of minus 1000 millivolts would reflect the approach of adangerous condition, at which the process should be temporarily stoppedor the current reversed for a short period. Where the reference halfcell 25 is a lead-lead oxide cell, a measurement of minus 500 millivoltswould reflect a danger level.

In order to measure accurately the reference voltage between thereinforcing bar 11 and the reference half cell 25, as by the voltagemeter V, it is necessary to interrupt the application of primarytreatment voltage from the external source 14. Thus, pursuant to anadvantageous procedure according to the invention, the external voltageis interrupted at periodic intervals, for example every ten minutes orso. As reflected in FIG. 3, when the external voltage is interrupted,the reference voltage dissipates along a curve 30, rapidly at first andthen more slowly as it approaches a limit condition representing thetrue reference voltage. After an interruption of five to ten seconds,the curve begins to flatten out and it becomes evident to the observerwhether the reference voltage is going to reach a predetermined voltagelevel 31, the value of which is a function of the composition of thereference half cell. In the illustration of FIG. 3, three interruptioncycles are illustrated. In the first, at the end of the interruptionperiod, the reference voltage is at a level indicated by the referencenumeral 32, which is comfortably above the predetermined danger level.Accordingly, the external voltage from the source 14 is reinitiated. Inthe second illustrated interruption cycle, approximately ten minuteslater, the reference voltage decay curve 33 approaches but still doesnot reach the danger level 31, and the external voltage is reestablishedfor a further cycle. At the end of the third illustrated cycle, thecurve 34 of the voltage decay is shown to pass below the line known toindicate a dangerous condition of the internal steel. At this time, theexternal voltage would either be discontinued for a sufficient period oftime to allow polarization of the steel to be dissipated, or theexternal voltage might be applied in the reverse direction for a shortperiod of time.

As will be readily appreciated, the procedure is readily subject towholly automatic control through simple microprocessor circuitry, forexample, designed to interrupt the external voltage on a predeterminedperiodical basis and to monitor the decay curve of the referencevoltage.

Where desired, of course, the half cell monitor may be utilized inconnection with any internally reinforced structure, whether or not theinternal reinforcement is under tension. However, the use of suchcontrols is considered highly significant in connection with tensionedreinforcement.

The procedure of the invention provides a particularly advantageous andefficacious method of removing excess chlorides from concrete structuresin a substantially non-invasive manner. The use of a self-adherent,removable material as an electrolytic surface medium enables andelectrolytic technique to be carried on vertical surfaces as well asdownwardly facing overhead surfaces. The self-adherent medium remainssufficiently moist to provide for adequate levels of conductivity, whileat the same time remaining adherent and cohesive during use and easilyremovable at the end of the procedure.

To particular and special advantage, the electrolytic medium iscomprised of a papier-mache-like material formed of cellulosic pulp,which typically may be newspaper grade pulp comprised of chopped pulpfiber. The pulp fiber may be virgin, but for cost purposes is moredesirably a recycled newsprint. The pulp material is mixed in situ withthe liquid and sprayed on a concrete surface, prepared by beingmoistened to a level of at least about 90% relative humidity. Thepapier-mache-like material, applied in situ, has important advantages tothe process, because of its high degree of self-adherence to theconcrete surface, which enables it to be easily utilized on verticaland/or overhead surfaces. Likewise, the material is inherently light inweight, which also facilitates its use on vertical and overhead surfacesas will be readily appreciated. Application of the papier-mache-likeelectrolytic medium to the surface of the concrete is simple andinexpensive, being carried out by means of a spray nozzle which servesto both mix and apply the material. Because of the inherent light-weightcharacter of the papier-mache material, it is typically desirable, whenusing such material, to separately support the external electrode grid,and this typically is done by means of battens of wood or otherrelatively non-conductive material.

The papier-mache-like material used for the electrolytic medium is easyto maintain. It may be remoistened easily by simply sprayingperiodically with additional water or other solution. It is also verydurable, in that it can easily be kept in place for the duration of thetreatment period required, typically two to eight weeks. Additionally,and of particular importance, the papier-mache-like material may bereadily cleaned off at the end of the treatment, using high pressuresprays, for example. Disposal of the used material is very simple andrelatively inexpensive.

For most purposes, the process of the invention advantageously utilizesan external grid electrode formed of steel, which is embedded in theadherent electrolytic medium. Utilizing a steel electrode grid, thechloride ions freed from the concrete and migrating into theelectrolytic medium cause the steel to corrode, thereby producingcorrosion products with the steel, rather than to emit free chlorinegas. For many situations, the release of substantial amounts of freechlorine gas could not be tolerated for safety reasons. The use of asteel electrode grid, while it causes the electrode to be consumed bythe corrosion products, is a superior solution to providing for ionexchange, for example. In most cases, the development of the corrosionproducts can be compensated for by increasing voltage levels (up to themaximum desired level of 40 volts). In particularly aggravated cases,the electrode grid may have to be replaced after a period of time,before the process has been completed. While the process of theinvention does not preclude the use of more conventional electrodematerials, such as platinized titanium, the use of steel electrodes ispreferred and advantageous for most circumstances.

In any of its various modifications, the process of the invention mayadvantageously utilize monitoring of the polarization of the internalsteel reinforcement as a significant control criterion. In this respect,because of a danger of hydrogen embrittlement, it would not have beenconsidered feasible, heretofore, to utilize a process such as thedescribed electrolytic procedure for chloride removal, in situationswhere the internal steel reinforcement was maintained under tension,particularly in the case of pre-tensioned or post-tensioned structures.In one of its aspects, the procedure of the invention, incorporatesmonitoring the condition of polarization through means such as the useof an embedded reference half cell. As polarization of the internalreinforcement increases, with continued practice of the process of theinvention, the polarization is periodically monitored. When it reaches alevel at which evolution of hydrogen gas is favored, and thereforehydrogen embrittlement of the steel in tension is likely to occur, theprocess is either terminated for a period of time or the polarity of theimpressed voltage is reversed for a short period, to effect dissipationof the condition of polarization. By this means, the process may besafely practiced in conjunction with tensioned structures.

It should be understood, of course, that the specific forms of theinvention herein illustrated and described are intended to berepresentative only, as certain changes may be made therein withoutdeparting from the clear teachings of the disclosure. Accordingly,reference should be made to the following appended claims in determiningthe full scope of the invention.

We claim:
 1. A process for rehabilitating internally reinforced concreteby electrolytic ion migration, which comprises(a) applying a removableadherent electrolytic coating to a surface area of said concrete, (b)impressing a DC voltage between the internal reinforcement and anelectrode structure associated with said electrolytic coating to effectmigration of negative ions from said concrete toward and into saidelectrolytic coating, (c) discontinuing said voltage and removing saidadherent coating when a desired level of ion migration has beenaccomplished, (d) periodically throughout said process measuring thepotential difference of said internal reinforcement against a referenceelectrode, and (e) temporarily discontinuing application of said DCvoltage at any time said potential difference indicates conditionsfavoring the evolution of hydrogen, leading to the possibility ofhydrogen embrittlement of said internal reinforcement.
 2. A processaccording to claim 1, wherein,(a) during said temporary discontinuanceof application of said DC voltage, a DC voltage of reverse polarity isapplied between said internal reinforcement and said electrodestructure.